G'day Marnie,
I am by no means an expert, but I think the generally accepted method for attaching your twisted leader to the main braided line is to use a loop-to-loop connection (do a search on the forum for instructions), which means you have to tie a bimini double in the main line. Personally, I only double about 30cm of the braid, and have never had any issues with that opening up during the cast and catching on a guide.
Regarding length, my twisted leaders are normally about 2 or 2.5 metres long, so I find that when rigged and ready to cast, the join of the leader is about 30cm away from my casting finger. Seems to work for me, and never had it fail, but others may do things differently.
I'm pretty sure a lot of the gurus have gone off twisted leaders in favour of heavy mono leaders joined with FG or PR knots, but seeing as I am defintely not a guru, I'll let them tell you about that.
Cheers
Angus